Glory boy Ndlovu came off the bench to grab the winner as Neil Warnock's United side carved out a hard earned victory over their South Yorkshire neighbours.
The game, a dreary goal spectacle for the fans, was grinding towards a goalless draw when Ndlovu made a timely intervention.
The Zimbabwean international came on after 71 minutes for John McGovern and made an immediate impact.
Ndlovu drifted into the Rotherham area unmarked and was perfectly placed when Canadian international Paul Peschisolido wriggled into the area and picked him out with a splendid pinpoint pass.
Ndlovu sized up the opportunity instantly and rifled in a low-angled shot from 15 yards to beat Rotherham keeper Mike Pollitt. Until then neither side had looked capable of breaking the deadlock.
The best efforts came in the dying minutes of the opening period.
McGovern was involved in most of them and found the head of Phil Jagielka three minutes before the break, but the young midfielder turned defender headed wide.
A McGovern cross was met by Carl Asaba but his effort went wide of the target.
Then in the dying seconds Stuart McCall picked out Aussie defender Shaun Murphy with a cross from the right but his powerful effort was finger-tipped over by the agile Pollitt.
Peschisolido who came on after 22 minutes for the injured Iffy Onuora did well early in the second half cutting in from the left before lashing a right-foot shot from a tight angle, but Pollitt denied him with a fine save.
After United's goal Rotherham at last looked for an opening, but the best they could offer was a poor free-kick smashed over by Martin McIntosh from 25-yards