The U's are now unbeaten in five games under Lewis, and they simply had too much firepower for the Stags as they consolidated their place in League Two's top three.
Connolly, on loan from Portsmouth, had found the net when coming off the bench in Oxford's previous home game against Wimbledon.
This time he was handed a start, and he repaid the faith of Lewis, who opted for a very attacking line-up that included four centre forwards, by opening the scoring with a low shot on the turn past keeper Alan Marriott.
Preston loanee Wroe then doubled the home side's lead with a superb 25-yard drive into the top corner in the 53rd minute.
Mansfield had been the better side until Connolly's breakthrough, but they could not beat goalkeeper Ryan Clarke.
Clarke made two smart saved from Stags' top scorer Sam Clucas in the first half, and after the break stopped efforts from Lee Stevenson and Ryan Tafazolli.
Smalley wrapped up a comfortable win by netting from close range in the 68th minute after James Constable's shot had come back off the post.
- Sports & Recreation
- David Connolly
- Nicky Wroe
- Deane Smalley