During the first full council meeting, in which councillors vote for a Lord Provost and to appoint the leader of the council and other key committee positions, George Redmond voted remotely.
One source said it was clear Mr Redmond was in an airport when he was voting remotely and suggested that he may have been travelling back from Seville.
Rangers lost on penalties to Eintracht Frankfurt in the final of the Europa League in the Spanish city on Wednesday night, with council sources saying Mr Redmond may have become stuck due to a cancelled flight following the match.
A Scottish Labour source confirmed the councillor had been in Seville.
In a piece for the Glasgow Times last week, Mr Redmond bemoaned the SNP leadership of Glasgow, saying Labour had the “energy, the drive and the hunger to be the champions that Glasgow desperately needs”.
Mr Redmond usurped former Labour leader in Glasgow, Malcolm Cunning, following the elections at the start of this month.
In a vote, the Mr Redmond beat Mr Cunning 24 votes to 11.
It followed Labour’s defeat in Glasgow in which it fell just short of becoming the largest party in Scotland’s biggest city, finishing one seat behind the SNP.
Mr Cunning had hoped to hold on to the leadership until mid-way through the council’s lifetime, but a leadership challenge from Mr Redmond succeeded despite his five-year break from the city chambers.