For the first quarter, we delivered revenues well above the high end of the range of our outlook with a total non-GAAP revenue ending the quarter at $257 million, representing year-over-year growth of 33%.
First-quarter adjusted EBITDA was $106.5 million, representing an adjusted EBITDA margin of 41%, exceeding the high end of our outlook for the first quarter.
Our Q1 core IT management maintenance renewal rate of 87% is higher than the low- to mid-80% renewal rates we noted we expected in 2021 when we discussed our full-year results in February.
We continue to focus on customer retention as a key priority and hope to grow back to our historical and best-in-class renewal rates of over 90%.
Our N-able business, again, delivered double digit, 13% revenue growth in Q1, and we continue to expand our portfolio in support of our MSP partners and SME customers.
We are accelerating our momentum in the database monitoring segment with the formation of a dedicated core team to help us capture what we believe is a large and growing market opportunity of over $6 billion.
First, the market opportunity for N-able starts with a small and medium enterprise IT spending, which is over $1 trillion globally and growing.
We believe the market opportunity for our software solutions is approximately $23 billion and expected to nearly double by 2025.
As we strive to become a stand-alone Rule of 50 company with a heavier lean toward growth from the potential spin-off transaction, I want to highlight the unique aspect of our growth model called partner enabled expansion.
We delivered a solid 13% total revenue growth and 15% subscription revenue growth, especially considering a couple of headwinds worth noting.
We had a much better quarter than anticipated and finished well above the high end of the range of our outlook for the first quarter with total non-GAAP revenue ending the quarter at $257 million, representing year-over-year growth of over 3%.
Total N-able revenue was $83 million, representing growth of 13%.
And total -- or core IT management revenue was $174 million.
Total license and maintenance revenue was $147.9 million in the first quarter, down 3.5% versus the prior year.
Maintenance revenue was $123 million in the first quarter, up 6% versus the prior year.
Our Q1 trailing 12-month rate was 91%.
However, given the heightened focus on smaller windows of performance since the cyber incident, we want to provide the in-quarter renewal rate for Q1, which was approximately 87%.
This exceeded our expectations for renewal rates in the low- to mid-80% range throughout 2021, which we provided on our last earnings call.
In addition, we expect this renewal rate of 87% to increase by a few percentage points based on historical trends after factoring in renewals that expire bookings that occur post quarter end.
For the first quarter, license revenue was $24.9 million, which represents a decline of approximately 33% as compared to the first quarter of 2020.
Total ARR reached approximately $961 million as of March 31, reflecting year-over-year growth of 12%.
Subscription ARR grew 13%, reaching $438 million at the end of the quarter.
First-quarter non-GAAP subscription revenue was $109.1 million, up 15% year over year, which was driven by N-able's 15% year-over-year subscription revenue growth, as well as solid performance in core IT management subscription revenue.
We finished the quarter -- we finished the first quarter of 2021 with 1,074 customers that have spent more than $100,000 with us in the last 12 months, which is a 16% improvement over the previous year and 17% more since year-end.
First-quarter adjusted EBITDA was $106.5 million, representing an adjusted EBITDA margin of 41%, exceeding the high end of the outlook for the first quarter.
Unlevered free cash flow for the first quarter totaled $51 million.
Excluded from EBITDA and unlevered cash flow -- unlevered free cash flow are one-time costs of approximately $20 million, including $10 million of spin-off-related costs and $10 million of cyber-related remediation, containment, investigation and professional fees, net of insurance proceeds.
They are separate and distinct from the $20 million to $25 million of Secure by Design initiatives, which are aimed at enhancing our IT security and supply chain process.
Net leverage at March 31 was 3.2 times our trailing 12-month adjusted EBITDA.
With $374.4 million in cash at March 31, we believe we are well-positioned from a financial standpoint to continue to invest in the future growth of our business.
For the second quarter of 2021, we expect total non-GAAP revenue to be in the range of $254 million to $258 million, representing year-over-year growth of 3% to 5%.
Adjusted EBITDA for the second quarter is expected to be $102 million to $104 million, which implies an approximately 40% adjusted EBITDA margin.
Non-GAAP fully diluted earnings per share is projected to be $0.21 per share, assuming an estimated 319.6 million fully diluted shares outstanding.
And last, our outlook for the second quarter assumes a non-GAAP tax rate of 22%, and we expect to pay approximately $22 million in cash taxes during the second quarter of 2021.
Increasing the percentage of our recurring revenue has been a focus over the past five years, and recurring revenue is down 90% of our total revenue.
We expect our conversion rate for the full year to be above 70% and grow to the low 80% range in 2022.

Summary:
Non-GAAP fully diluted earnings per share is projected to be $0.21 per share, assuming an estimated 319.6 million fully diluted shares outstanding.