Harry Styles, Liam Payne and Niall Horan were all on hand to watch Tomlinson's mum Johannah Poulston exchange vows with her partner Daniel Deakin in their hometown of Doncaster, England.
While fellow bandmate Zayn Malik was not at the nuptials, Tomlinson's longtime girlfriend, Eleanor Calder, served as one of Poulston's bridesmaids.
Horan and Payne, who was joined by his girlfriend Sophia Smith, were spotted playing croquet at the reception.
Poulston and Deakin have been dating for two years, and in February, they welcomed twins, Ernest and Doris.
Poulston split from her famous son's father Mark Tomlinson while the 22-year-old was competing on The X Factor in 2010.