Former Republic of Ireland striker David Connolly struck in the 27th minute, and Nicky Wroe and Deane Smalley added further second half goals as the U's equalled their biggest home win of the season and stretched their unbeaten run to eight games.
Lewis said: "We decided to go with four forwards to go and win the game, and this result makes our three away draws we've just had now seem even better results.
"I talked it over with Andy Melville and my coaching staff and we just thought 'let's go for it'.
Yet Mansfield could look back on missed chances as they had plenty of the game, but lacked a killer touch in front of goal.
It was Lewis's 49th birthday, so a third successive home win proved a pleasant birthday present.
He said: "I was trying to keep that quiet really. We rode our luck a bit in the first half, and the wind was awful so it was hard to get out of our own half.
"But lads have put their bodies on the line, Ryan Clarke made two great saves, and then David Connolly showed his quality. He was great all day, he just keeps the ball for you.
"All the goals were good goals, and the third goal showed the quality we have got."
Lewis added: "You can see when the first goal went in how the confidence is lifted.
"The players are a fabulous bunch to work with. It was a really good team performance. It's lovely to have three wins on the bounce at home, let's build on that."
- Sports & Recreation
- David Connolly
- Deane Smalley
- Dave Kitson
- Nicky Wroe