The host, 42, has been on her usual summer break for the past few weeks, enjoying a vacation with her television executive husband Dan Baldwin and their three children.
They confirmed Willoughby would be joined by Alison Hammond for hosting duties.
And on Monday morning, the show began with a live performance from Mamma Mia! The Party - an experience which takes place at London’s 02. Visitors take a trip to the The Island Of Skopelos and enjoy a night at Nikos’ Taverna, which descends into a Turns Into A Fabulous Disco after dinner.
On Monday, Willoughby and Hammond could be seen dancing and singing away with other This Morning regulars such as James Martin and Gyles Brandreth - outside the ITV studios to various hits from the show.
The presenter made reference to the fact her children had returned to school today as the weather was expected to hit highs of 28C in London.
Willoughby then teased a number of features coming up on This Morning this autumn before cracking on with Monday’s show.
She also teased her outfit half an hour before the show began on her Instagram page.
It is not yet clear whether Hammond —who is the new co-host of The Great British Bake Off — will join Willoughby as permanent co-host though she is expected to be by Willouughby’s side this week.
Willoughby is expected to lead the show Monday to Thursday with Hamond and Dermot O’Leary predicted to continue on their usual Friday slot.
Their return to the small screen comes ahead of the National Television Awards (NTAs) on Tuesday, which will see This Morning stars Hammond and Martin Lewis up for the best TV presenter prize.
Earlier this year, Phillip Schofield departed his long-time This Morning co-presenting role after admitting to an affair with a younger male colleague.
Schofield, 61, resigned from ITV and was dropped by his talent agency YMU in May after admitting to the “unwise but not illegal” relationship.
Holly Willoughby had hosted This Morning alongside Schofield from 2009 onwards.
Despite the scandal,This Morning has been nominated for best daytime show at the National Television Awards — which take place next week.
Schofield and Willoughby had been longlisted for the best TV presenter prize, alongside fellow This Morning hosts Hammond, O’Leary,Rylan Clark and Martin Lewis, but only Hammond and Lewis made the cut on the shortlist.
Willoughby made a brief return to This Morning last month to promoteher cameo on ITV drama Midsomer Murders, which screened on Sunday.
Teasing her role, Willoughby shared some behind-the-scenes footage, which airs on Friday’s This Morning, on her Instagram a day early and said that being on the series had been a “dream come true”.