Aside from a momentary scare when Scunthorpe, who fancy themselves as a footballing side, had the front to equalise midway through the first half, this was a cruise for City, who may be regarded now as a force in the game but whose appearance in the quarter-finals of the Carling Cup will be only their fourth in the competition in 20 years.
Jonathan Forte cancelled out Stephen Ireland's third-minute opener but Roque Santa Cruz, Joleon Lescott, Carlos Tevez and Michael Johnson ultimately swamped the Championship side and took City a step nearer their first silverware since 1976, although seven of the current top eight in the Premier League are also in the hunt.
"I felt we were still in it at half-time," Scunthorpe manager Nigel Adkins said. "But their quality shone through in the second half. They have set out to win the competition and I'm pleased we have scored a goal and created some chances."
His ambitions in the competition not withstanding, City manager Mark Hughes still made six changes from the side that let a two-goal lead slip against Fulham last weekend. Two were enforced, with Kolo Touré and Martin Petrov injured, but Micah Richards, Wayne Bridge and Craig Bellamy dropped to the bench, with Emmanuel Adebayor not involved at all.
Hughes had prefaced the match by batting away suggestions of defensive frailty and a goal in the third minute seemed to quash any notion that this City line-up might be vulnerable. Shaun Wright-Phillips released Ireland on the left and the midfielder stroked the ball effortlessly beyond Joe Murphy's reach.
City were jolted before they took control. Scunthorpe may have been playing two levels below them last season but they have a reputation for confident football and when Nigel de Jong gave the ball away carelessly in the centre of the field after 25 minutes they took advantage, combining in a swift exchange of passes that culminated in Marcus Williams squaring the ball from the left for Forte to slide home from close range.
Paul Hayes might have put the visitors in front soon afterwards had his header, from a Forte cross, not gone straight at Shay Given but City, to their credit, came up with the response Hughes would have wanted, quickly finding their composure.
Wright-Phillips drew a good save from Murphy but the goalkeeper had little chance against Santa Cruz as Pablo Zabaleta delivered a cross from wide on the right, the Columbian striker losing his marker at the far post, from where his diagonal header found the opposite corner of the net as he marked his first start for City with his first goal.
With their lead restored, City asserted themselves in the second half. After Murphy had denied Santa Cruz, and David Mirfin had taken the ball off Ireland's head with the goal gaping, Lescott's header from the resulting corner gave City a two-goal advantage on 56 minutes, which was extended after 71 when Tevez converted substitute Vladimir Weiss's cross.
Johnson, who played the last 18 minutes, fired in from 20 yards. It was the home-grown midfielder's first goal in more than two years after a long struggle with injury. "Michael's goal is a great lift for everyone at the club," Hughes said. "He has been out for too long, but if we can keep him fit and well now he will be like a new signing for us."
Manchester City (4-4-2): Given; Zabaleta, Kompany, Lescott, Sylvinho; Wright-Phillips, De Jong, Barry (Johnson, 72), Ireland (Weiss, 59); Santa Cruz, Tevez (Mwaruwari, 79). Substitutes not used: Richards, Bridge, Taylor (gk), Bellamy
Scunthorpe United (4-4-2): Murphy; Byrne, Mirfin, Jones (Canavan, 46), Williams; O'Connor (Hooper, 67), J Wright, McCann (Togwell, 77), Woolford; Hayes, Forte. Substitutes not used: A Wright, Sparrow, Spence, Lillis (gk)
Referee: M Oliver (Northumberland)