The Australians didn't lose a wicket in a potentially series-defining second session at Old Trafford on Thursday, leaving them favorites to claim a win and guarantee retaining the urn.
Smith was on his way back to the pavilion, berating himself, after edging Leach to Ben Stokes at slip when on 118. The world's No. 1 test batsman was given a life, however, when replays showed Leach had overstepped — an embarrassment for a spinner.
It wasn't long before Smith was becoming the first Australian since Allan Border in 1981-85 to score more than 500 runs in consecutive Ashes series and only the third player — after Don Bradman and Jack Hobbs — to post that number in three Ashes series.
Captain Tim Paine was alongside Smith on 58, having been dropped twice — on 9 and 49. Both were straightforward chances, with Jason Roy spilling the ball at second slip off Stuart Broad and Sam Curran putting it down at midwicket off Jofra Archer.
Curran was only on the field because Stokes was forced to quit midway through an over he was bowling, due to soreness in his right shoulder. He was only briefly off the field.
It was Paine's first half-century of the series, and in nearly a year in tests.
Smith struck 144 and 142 in the first test at Edgbaston, and then 92 in the first innings of the second test at Lord's before delayed concussion — felt after being struck on the neck by a bouncer from Jofra Archer — meant he didn't bat in the second innings.
He missed the third test, won by England at Headingley.
Back in the team, Smith — jeered relentlessly in England owing to his involvement in a ball-tampering scandal in South Africa last year — even received a round of applause from fans in Manchester after getting to triple figures with a clip off the pads for two.
It is Smith's fifth century in his last eight innings against England, and he has struck 20 fours in his 263-ball knock. His highest test score is 239, against England in Perth in 2017.
After Australia resumed on 170-3 following a rain-affected opening day, Travis Head (19) was trapped lbw by Broad from around the wicket, giving the paceman a third wicket of the innings.
Matthew Wade departed for 16 after slogging Leach and getting caught at mid-on.
The teams are 1-1 with two matches left. Australia will retain the urn if the series is drawn.