The on-loan Nimely, who spent the first half of the season with Middlesbrough, set up Gary McSheffrey for the game's opening goal and added a second early in the second half after Kevin Thomson had been sent off.
Middlesbrough captain Matthew Bates then scored at the wrong end after a clever cross from Coventry substitute David Bell.
Scott McDonald pulled one back for the visitors who made three quick substitutions to try and turn the game in their favour.
But the hosts held on with Boro winger Julio Arca seeing red in the closing stages for a two-footed lunge on Sammy Clingan.
Middlesbrough's new signing Lukas Jutkiewicz was denied the chance to face his former employers due to an agreement between the two clubs.
But fellow striker McDonald was presented with two glorious opportunities inside the opening nine minutes.
He latched onto a mistake from Sammy Clingan only to volley straight at Joe Murphy before the Sky Blues stopper produced a magnificent save to deny the same player moments later.
Murphy continued to impress as another flowing Middlesbrough move culminated in a Joe Bennett drive which was tipped over by the Republic of Ireland international.
The dynamics of the game then shifted significantly when the hosts broke the deadlock in the 36th minute as Nimely fed McSheffrey to slot home from inside the area.
The visitors were reduced to 10 men moments later when Thomson, who had been harshly booked just 11 minutes earlier for bringing down Cyrus Christie, was shown a second yellow card.
The midfielder could have no complaints over his second caution as he flew in with his studs showing to bring down goalscorer McSheffrey.
The game threatened to boil over in the aftermath of Thomson's dismissal and that prompted Coventry boss Andy Thorn to substitute Gary Deegan, on a yellow card, for Bell at half-time.
Nimely, making his first appearance since joining on loan from Manchester City, failed to score during his time with Boro but he headed home 57 minutes into his Coventry career.
Boro felt the ball had not crossed the line as a defender hooked it clear but the assistant referee was perfectly placed to award the goal.
Nimely was involved in the third goal seven minutes later as he released Bell out wide and his first time cross was touched in by Bates with Clive Platt in close pursuit.
Boro responded instantly when Marvin Emnes' driven cross was nodded in by McDonald and Mowbray attempted to rally his troops with a triple change.
But Coventry used their extra man advantage to close out the game as Middlesbrough's dismal afternoon was completed in stoppage time when Arca earned their second red card.