It came as election chiefs confirmed that the next mayor will be declared tonight, with the result expected around 8.30pm.
Mr Khan polled 67,610 first preference votes in the north London constituency, against 65,822 for Mr Bailey, who was down almost 8,000 votes on the number polled by Tory candidate Zac Goldsmith in 2016.
Mr Khan also scored a hefty 76,731 first-preference votes in the Labour heartland of Greenwich and Lewisham, compared with 43,306 for Mr Bailey.
And he was almost 17,000 votes ahead of Mr Bailey in Merton and Wandsworth, polling 76,403 first preference votes to 59,460 for Mr Bailey.
Here Mr Bailey also failed to match the performance of his Tory predecessor in 2016 by about 13,000 votes.
Shortly before 7pm, Mr Khan claimed a fourth successive constituency, outpolling Mr Bailey in Enfield and Haringey, by 74,646 votes to 48,101.
Though Mr Khan was 13,000 votes down in the constituency on his performance in 2016, and Mr Bailey was up 3,000, the gap of 26,545 in favour of Mr Khan is another big step to eventual victory.
The votes received in all 14 constituences are added up to determine the winner. It is highly likely that second preference votes will have to be counted. These come into play if no candidate secures more than 50 per cent of first-preference votes.
Earlier this afternoon, Mr Khan opened up a five point lead over Shaun Bailey. By 6pm he had retained the lead.
By 2.30pm, with counting three-quarters complete, Mr Khan was on 40 per cent but Mr Bailey had fallen back one point to 35 per cent.
The first day’s counting ended last night with Mr Khan having received 487,104 first preference votes to 462,837 for Mr Bailey – a lead of 24,267.
Counting resumed today at 8am.
By 11am today, Mr Khan was ahead in five of the seven remaining constituencies and had an overall lead of 40 per cent to 36 per cent for Mr Bailey.
By 2.30pm, Mr Khan had retained his lead in the same five constituencies.
Mr Khan was ahead in Barnet & Camden – though only by one point after Mr Bailey narrowed his lead in the marginal north London constituency to 39 per cent v 38 per cent – and in City & East (49/27), Enfield & Haringey (46/30), Greenwich & Lewisham (47/26) and Merton & Wandsworth (44/33).
Mr Bailey was ahead in Croydon & Sutton (45/31) and South West (37/35). He had slowly gained ground throughout the morning but looked unlikely to gain sufficient votes to overtake Mr Khan.
London Elects, the organisation running the election, confirmed at 6pm tonight that the next mayor would be announced around 830pm . There had been fears that the declaration would be delayed until Sunday.
Any ballot papers marked with their name as the voter’s second preference are then added to their total.
Seven of the 14 constituencies on the London Assembly were declared yesterday. None changed hands – the Tories held three and Labour four.
But the results for the mayor from each constituency in general showed Mr Bailey doing better than expected and Mr Khan less well than in 2016, when he first became mayor by defeating Tory candidate Zac Goldsmith.
The seven constituencies declaring today were always more likely to be favourable to Mr Khan, as five of these returned a Labour majority in the 2016 mayoral and London Assembly elections.
The constituencies declaring today are: City and East, Croydon and Sutton, Barnet and Camden, Greenwich and Lewisham, Merton and Wandsworth, Enfield and Haringey and South West.
The final 11 assembly seats, which are determined according to a complex proportional representation formula based on party support across London, are due to be announced from 9.30pm tonight.
The first results on Friday sent social media into meltdown as they suggested that Mr Bailey could be in with a chance of pulling off arguably the most dramatic mayoral result seen since the post was established in 2000.
Results from the first four constituencies put him 5,307 votes ahead of Mr Khan.
He also pulled off a surprise lead in Brent and Harrow, with 65,566 votes to 61,778 – a gap of 3,788.
And he was ahead in the West Central area, which covers key Tory areas such as Westminster and Kensington and Chelsea, including the Ladbroke Grove area where Mr Bailey grew up. Here he had a lead of 2,205, poling 53,713 to 51,508 for Mr Khan.
But Mr Khan scored a thumping majority of 56,966 first preference votes in the Labour stronghold of Lambeth and Southwark.
The votes for the mayoral candidates were published in the small print of each of the results for the seven London Assembly seats declared.
They showed that Mr Bailey exceeded expectations by some distance. In Bexley and Bromley he polled more than 100,000 votes – more than 3,000 more than Zac Goldsmith managed for the Tories in 2016.
In Brent and Harrow, the assembly seat was retained by Labour with a majority of more than 20,000 votes – but Mr Bailey outperformed his party’s assembly candidate by about 9,000 votes.
He also outperformed Mr Khan by 5,009 votes in Ealing and Hillingdon, with 79,863 votes to 74,854. This was in spite of the seat being held by Labour on the London Assembly.
Mr Bailey racked up a storming 82,361 votes in Havering and Redbridge, the constituency he and his family now call home, compared with 49,818 for Mr Khan – a net win of 32,543.
But Mr Khan performed strongly in the North East constituency, polling 67,126 more votes than his rival, 111,359 to 44,233.
In both of these east London constituencies, Mr Bailey beat the Tory vote achieved by Zac, now Lord, Goldsmith, in 2016 by several thousand votes – despite a fall in overall turnout.
The results also showed the Greens performing strongly, claiming second place in the Lambeth and Southwark assembly constituency and putting them on course for three or a record four assembly members.
The Lib-Dems hoped their results would improve today when seats such as South West declare. However mayoral candidate Luisa Porritt was at risk of losing her deposit.
The job of the 25-member assembly is to hold the mayor to account and scrutinise his or her policies.