Adjusted EBITDA1 increased 74.4% to $54.5 million, compared to $31.2 million in the prior year. The increase in Adjusted EBITDA1 was primarily due to higher revenues and favorable shifts in business mix. Adjusted EBITDA margin1 improved 320 basis points to 16.6%, compared to 13.4% in the prior year period.
Cash provided by operating activities for the full year ended December 31, 2020 was $1.9 million, compared to cash provided by operating activities of $17.0 million in the prior year. The change between periods mainly reflects an increase in working capital investments of $12.8 million, higher contingent earnout payments of $6.4 million, higher non-capitalized IPO and secondary offering-related costs of $7.0 million, and higher interest payments of $6.1 million. Excluding acquisition-related contingent earnout payments and non-capitalized IPO-related costs, which are not part of recurring operations, cash flow provided by operating activities decreased $1.8 million versus the prior year, primarily due to higher receivables caused by significant fourth quarter revenue growth. Overall days sales outstanding at December 31, 2020, remained unchanged compared to the prior year.
Liquidity and Capital Resources
In October 2020, Montrose improved its borrowing costs through the repricing of its $175.0 million term loan facility which matures in 2025. The interest rate spread on the term loan was reduced by 50 basis points to LIBOR, with a 1.00% floor, plus 4.50%, compared to the prior rate of LIBOR, with a 1.00% floor, plus 5.00%.
At December 31, 2020, Montrose had total debt, net of deferred debt issuance costs, of $175.9 million and $84.4 million of liquidity, including $34.4 million of cash and $50.0 million of availability on its revolving credit facility. As of December 31, 2020, the Company’s leverage ratio under its credit facility, which includes the impact of acquisition-related contingent earnout payments that may become payable in cash, was 2.7 times. Excluding contingent earnout payments of $4.4 million related to CTEH estimated 2021 earnings, an amount which may vary given the environmental emergency response nature of their work, the Company’s leverage ratio was 2.6 times.
Acquisitions
In January 2021, Montrose acquired MSE Group (“MSE”), a leading provider of environmental services primarily to the U.S. federal government. The addition of the MSE team is strategically additive to our Remediation and Reuse Segment, increases our environmental service offerings for select U.S. federal agencies, and expands our geographic presence in the Southeast U.S. The Company’s M&A pipeline and outlook for deal activity in 2021 remain strong.
Full Year 2021 Outlook
The Company is introducing its full year 2021 outlook for Adjusted EBITDA1 to be in the range of $61 million to $67 million, reflecting anticipated year-over-year revenue growth of approximately 20% at the mid-point. The Company expects Adjusted EBITDA margin1 to be in the range of 16.0% to 17.0% for full year 2021.
The current outlook is based on a combination of mid-to-high single digit organic growth plus the contribution of completed acquisitions. The outlook does not include any benefit from future acquisitions that have not yet been completed.