Steven Gerrard's men capitalised after reigning champion Celtic, the Gers' bitter Glasgow rival, failed to hold on to a lead earlier when Aberdeen snatched an injury-time equaliser in a 3-3 draw at Pittodrie.
Rangers have established a clear gap between themselves and second-placed Celtic, which is bidding to become the first side to win 10 successive Scottish titles. Both the Hoops (also in 1965-74) and Rangers (1988-97) have previously managed nine in a row.
After Joe Aribo put Rangers in front at Ibrox, Defoe struck in the 16th minute with his landmark goal, the much-travelled 38 year-old's first in league football this season.
The former England striker, who scored his first goal in senior football with West Ham 20 years ago last month, received a ball over the top from James Tavernier, got in between Livingston's centre-backs and did not break stride with a left-foot finish.
Defoe said he was thankful the landmark had slipped his mind: "Before the game I actually forgot, which to be honest was a good thing because if I had been thinking about it during the game maybe that goal wouldn't have come, maybe I would have snatched at the chance."
As for pulling further clear of Celtic, a cautious Defoe insisted: "It's important we just focus on ourselves, take each game as it comes, keep working hard, keep believing that we can improve.
"We can't take our foot off the gas, just keep going."
Earlier, Celtic manager Neil Lennon said referee Willie Collum had had a "very poor game" after his side was denied victory by Lewis Ferguson's late penalty.
Celtic, which came into the match after consecutive defeats to Rangers and then Serie A leader AC Milan in the Europa League, twice came from behind only to be denied.
Ferguson opened the scoring with his first penalty two minutes before half-time. Callum McGregor equalised in the 52nd minute.
Ryan Hedges restored Aberdeen's lead in the 64th minute but Celtic substitute Leigh Griffiths equalised in the 75th minute. The Dons were reeling when the Hoops' Ryan Christie scored a penalty two minutes later.
There was more spot-kick drama, however, when McGregor tripped Connor McLennan and Ferguson struck another precise penalty to give fourth-placed Aberdeen a point.
"I'm disappointed we haven't won it," Lennon said. "We showed the character to come back twice and we then lose that late goal. Defensively, we were poor but attacking it was good, particularly in the second half. It's too sloppy defensively though."
"I thought the referee had a very poor game today, decision making, being influenced by his assistants and some of his decisions were baffling," the Hoops boss added.