Non-interest income totaled $2.1 million for the three months ended September 30, 2020; an increase of $637,000, or 43.7%, from the comparable period in the prior year. The increase was primarily due to an increase in the net gain on the sale of loans, which increased $817,000, or 513.8%, and a $155,000 increase in investment advisory income. The gain was partially offset by a $171,000 decrease in service charges on deposit accounts and a $208,000 decrease in other non-interest income. Non-interest income increased $1.3 million, or 30.1%, to $5.4 million for the nine months ended September 30, 2020. In the nine months ended September 30, 2020, net gain on the sale of loans increased $2.0 million, or 549.0%, while investment advisory income increased $175,000, or 22.8%. These increases were offset by a $436,000 decrease in service charges on deposit accounts and a $541,000 decrease in other non-interest income due primarily to the decline in loan servicing income.
For the third quarter of 2020, non-interest expense increased $600,000, or 8.8%, to $7.4 million over the comparable 2019 period. The increase was primarily due to an increase in salaries and benefits of $214,000, an increase in FDIC deposit insurance of $219,000, and a $218,000 increase in other non-interest expense. For the nine months ended September 30, 2020, non-interest expense increased $698,000, or 3.4%, to $21.5 million over the comparative nine-month period in 2019. Salaries and benefits increased $390,000, or 3.3%, which was primarily attributable to annual merit increases, production incentives and employee benefit expense increases. FDIC deposit insurance increased $296,000, or 93.4%, due to an assessment credit received in the prior year and other noninterest expense increased $104,000, or 3.1%.
Balance Sheet Analysis
Total assets were $1.11 billion at September 30, 2020, representing an increase of $138.6 million, or 14.2%, from $973.9 million at December 31, 2019. Cash and due from banks increased $39.6 million from December 31, 2019, to $51.6 million, primarily due to an increase in deposits held at the Federal Reserve Bank of New York. Net loans increased $97.0 million, or 12.2%, and included $89.6 million of outstanding SBA PPP loan balances and a $12.5 million, or 3.4%, increase in our net indirect automobile portfolio. Other assets also include the right-of-use asset (“ROUA”) of $6.4 million at September 30, 2020 due to the current year adoption of the Accounting Standards Update 2016-02, Leases (Topic 842).
Past due loans decreased $4.1 million, or 23.0%, between December 31, 2019 and September 30, 2020 finishing at $13.6 million, or 1.5% of total loans, down from $17.6 million, or 2.2% of total loans, at year-end 2019. During the same timeframe, non-performing assets decreased $3.4 million or 32.4%, to $7.0 million. Our reserve as a percentage of total gross loans was 1.18% at September 30, 2020 as compared to 0.75% at December 31, 2019.
During the first nine months of 2020, total liabilities increased $133.3 million, or 15.4%, to $997.4 million, mainly due to a $142.9 million increase in deposits due to the inflow of cash from PPP loans and an apparent flight to safety as investors fled the stock market volatility. The lease liability, which offsets the ROUA, was $6.4 million at September 30, 2020 and also contributed to the increase. Decreases of $11.4 million in Federal Home Loan Bank advances and $4.1 million in mortgagors’ escrow accounts partially offset the increase in liabilities.
Stockholders' equity increased $5.3 million to $115.2 million at September 30, 2020, primarily due to net income of $3.6 million and a $1.6 million increase in the net unrealized gain on available for sale securities. The Company's ratio of average equity to average assets was 10.60% for the nine months ended September 30, 2020 and 11.34% for the nine months ended September 30, 2019.