The Spanish head coach was linked with Manchester United as a potential replacement for Sir Alex Ferguson in 2013 but ultimately agreed to take over the reins of Bayern Munich.
Guardiola succeeded Manuel Pellegrini at The Etihad Stadium in 2016 and led Manchester City to a third-place finish in his first season in charge of the Premier League club.
The Manchester City boss has a bitter rivalry with Manchester United manager Jose Mourinho from their respective spells in charge of arch rivals Barcelona and Real Madrid.
Guardiola’s side are 11 points clear of second-placed Manchester United after David Silva and Nicolas Otamendi scored in a 2-1 victory over their title rivals in Sunday’s derby at Old Trafford.
Manchester United have since bounced back with successive wins against Bournemouth and West Brom, while Manchester City eased to commanding victories over Swansea City and Tottenham Hotspur.
Manchester City set a new Premier League record with 14 successive victories in a single season following their triumph at Manchester United and their winning streak now stands at 16 triumphs in a row.
Former Citizens defender Mills believes Manchester United would have an even bigger lead than Manchester City right now if Guardiola was in charge at Old Trafford instead of The Etihad.
“I think Man United would be further ahead [under Pep]. Who would Man City have?” Mills told BBC Radio 5 Live.
Manchester United haven’t won the Premier League title since 2013 under Sir Alex Ferguson.
The Red Devils finished in sixth place in Mourinho’s first season in charge but won the Europa League and the League Cup.
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