Exhibit 99.1

S&T Bancorp, Inc. Announces Third Quarter Earnings

Indiana, Pennsylvania – S&T Bancorp, Inc. (NASDAQ: STBA), a full-service financial institution with office locations in 10 Pennsylvania counties, has announced its third quarter earnings.

Todd D. Brice, president and chief executive officer of S&T Bancorp, Inc., offered the following highlights:

“At S&T Bank, our recent agreement to acquire Mainline Bancorp, Inc. will enable us to gain greater market share and strengthen our existing footprint in Cambria and Blair counties, thereby having a positive impact on future earnings,” Brice said. “In addition, we are pleased to announce another quarter of strong earnings. In particular, our nonperforming assets are at the lowest levels since December of 2008, which demonstrates our continued progress in resolving challenging loans within our portfolio.”

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S&T Earnings Release - 2

S&T Bancorp, Inc. Announces Third Quarter Earnings (cont.)

Net Interest Income

Net interest income and net interest margin (FTE) decreased from the second quarter mainly due to a continued shift in asset mix to lower yielding assets. Net interest income on a fully taxable equivalent (FTE) basis for the third quarter of 2011 was $34.9 million compared to $35.6 million in the second quarter of 2011 and $38.1 million in the third quarter of 2010. The net interest margin (FTE) was 3.76% in the third quarter of 2011 compared to 3.85% in the second quarter of 2011 and 4.09% in the third quarter of 2010. Average loans decreased $76.6 million from the second quarter of 2011, while the fully taxable-equivalent yield decreased by 4 basis points to 4.85%. The average securities balance increased by $51.9 million mainly due to an increase in excess funds at the Federal Reserve. The fully taxable-equivalent yield on average securities decreased by 37 basis points to 2.39%.

Asset Quality

Asset quality metrics continue to show signs of improvement. Nonperforming assets decreased to $65.2 million or 2.08% of total loans plus other real estate owned (“OREO”) compared to $69.9 million or 2.18% in the second quarter of 2011 and from $82.7 million or 2.45% in the third quarter of 2010. Included in nonperforming assets were troubled debt restructurings (“TDRs”) of $22.1 million. Loans transferred to held for sale in the second quarter 2011 of $8.8 million were sold during the third quarter of 2011 and resulted in an additional $0.2 million loss on loans held for sale within noninterest expense.

The provision for loan losses was $1.5 million for the third quarter of 2011 compared to $1.1 million for the second quarter of 2011 and $8.3 million in the third quarter of 2010. Net charge-offs were $8.0 million for the third quarter of 2011 compared to $4.8 million in the second quarter of 2011 and $6.0 million in the third quarter of 2010. Approximately $6.0 million of the $8.0 million of net loan charge-offs this quarter had previously established specific reserves. At September 30, 2011, the allowance for loan losses was $51.5 million or 1.64% of total loans, as compared to $58.0 million or 1.81% at June 30, 2011 and $56.3 million or 1.67% at September 30, 2010.

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S&T Earnings Release - 3

S&T Bancorp, Inc. Announces Third Quarter Earnings (cont.)

The allowance to nonperforming loans was 87% at September 30, 2011, 93% at June 30, 2011 and 75% at September 30, 2010. Included in the allowance is $2.9 million of specific reserves compared to $7.5 million in the second quarter of 2011 and $10.5 million in the third quarter of 2010.

Noninterest Income and Expense

Noninterest income was $10.4 million for the third quarter of 2011 compared to $11.2 million in the second quarter of 2011 and $12.3 million in the third quarter of 2010. The decrease from the prior quarter primarily relates to a $0.8 million impairment charge for mortgage servicing rights compared to $0.2 million for the second quarter of 2011. This impairment charge reflects a decline in the value of the remaining mortgage servicing rights due to increased prepayment speeds resulting from a decrease in interest rates during the quarter. Positively impacting noninterest income for the quarter was $0.3 million of new deposit fees.

Noninterest expense for the third quarter of 2011 was $24.2 million compared to $25.6 million for the second quarter of 2011 and $24.9 million for the third quarter of 2010. The decrease from the prior quarter in noninterest expense primarily relates to a decrease in the unfunded commitment reserve of $1.1 million. Approximately $0.8 million of the decline related to an expense recognized in 2008 for a letter of credit that S&T was contractually obligated to fulfill. During the third quarter of 2011, the letter of credit was drawn upon and funded and a corresponding loan charge-off was recorded. In addition, the unfunded commitment reserve decreased $0.4 million due to a decline in commitment levels.

Financial Condition

Total assets were essentially unchanged in the third quarter of 2011 compared to the second quarter of 2011 and the third quarter of 2010. Total portfolio loans were $3.1 billion at September 30, 2011 compared to $3.2 billion at June 30, 2011 and $3.4 billion at September 30, 2010. Total portfolio loans have decreased $59.7 million from the prior quarter including $6.1 million in consumer loans and $53.6 million in commercial loans.

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S&T Earnings Release - 4

S&T Bancorp, Inc. Announces Third Quarter Earnings (cont.)

The decline in loan balances is a result of a slow economic recovery resulting in businesses and consumers remaining cautious and significant loan principal pay-downs.

Total deposits were essentially unchanged in the third quarter of 2011 compared to the second quarter of 2011 and the third quarter of 2010. Noninterest-bearing demand deposits remain strong at $817.5 million as of September 30, 2011 compared to $801.5 million at June 30, 2011 and $743.5 million at September 30, 2010.

S&T’s capital ratios continue to improve due to a reduction in risk-weighted assets and earnings retention. S&T’s capital ratios exceed the “well capitalized” thresholds of federal bank regulatory agencies with a tier 1 leverage ratio of 11.80%, tier 1 risk-based capital ratio of 14.95% and total risk-based capital ratio of 18.51% at September 30, 2011. This compares to tier 1 leverage ratio of 11.49%, tier 1 risk-based capital ratio of 14.31% and total risk-based capital ratio of 17.83% at June 30, 2011. S&T’s tangible common equity ratio for the third quarter of 2011 was 8.30% compared to 8.08% for the second quarter of 2011 and 7.53% for the third quarter of 2010.

Other Highlights

On September 14, 2011, S&T Bancorp, Inc. and Mainline Bancorp, Inc. jointly announced the signing of a definitive merger agreement pursuant to which S&T Bancorp, Inc. will acquire Mainline Bancorp, Inc. in a stock and cash transaction valued at approximately $21.5 million. Based in Ebensburg, Pennsylvania, Mainline Bancorp has assets of $241.8 million and maintains eight branches. The transaction is in-market and will expand S&T Bancorp, Inc.’s existing footprint in Cambria and Blair counties, Pennsylvania. The combined company will have more than $4.3 billion in assets. The transaction is expected to be completed in the first quarter of 2012, after satisfaction of customary closing conditions, including regulatory approvals and the approval of the shareholders of Mainline Bancorp, Inc.

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S&T Earnings Release - 5

S&T Bancorp, Inc. Announces Third Quarter Earnings (cont.)

About S&T Bancorp, Inc.

Headquartered in Indiana, PA, S&T Bancorp, Inc. operates offices within Allegheny, Armstrong, Blair, Butler, Cambria, Clarion, Clearfield, Indiana, Jefferson and Westmoreland counties. With assets of $4.1 billion, S&T Bancorp, Inc. stock trades on the NASDAQ Global Select Market under the symbol STBA. For more information, visit www.stbancorp.com.

This information may contain forward-looking statements regarding future financial performance which are not historical facts and which involve risks and uncertainties. Actual results and performance could differ materially from those anticipated by these forward-looking statements. Factors that could cause such a difference include, but are not limited to, general economic conditions, change in interest rates, deposit flows, loan demand, asset quality, including real estate and other collateral values and competition. In addition to the results of operations presented in accordance with Generally Accepted Accounting Principles (GAAP), S&T management uses and this press release contains or references, certain non-GAAP financial measures, such as net interest income on a fully taxable equivalent basis. S&T believes these non-GAAP financial measures provide information useful to investors in understanding our underlying operational performance and our business and performance trends as they facilitate comparisons with the performance of others in the financial services industry. Although S&T believes that these non-GAAP financial measures enhance investors’ understanding of S&T’s business and performance, these non-GAAP financial measures should not be considered an alternative to GAAP. A reconciliation of these non-GAAP financial measures is presented in the attached consolidated selected financial data spreadsheet. This information should be read in conjunction with the audited financial statements and analysis as presented in the Annual Report on Form 10-K for S&T Bancorp, Inc. and subsidiaries.

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