The pair stood side by side and chatted outside the church before going in to attend the service for the Marquess of Londonderry, who died in June.
They also greeted fellow churchgoers, who included Princess Michael of Kent and TV presenter Kirstie Allsopp.
Fellow guests at St Paul's church in Knightsbridge included Jemima Khan, Ben's sister. She attended with her sons Sulaiman and Qasim, from her marriage to Imran Khan.
Ben and 29-year-old Kate appear to have smoothed things over after they fell out over her relationship with American rapper Jay Electronica.
News emerged that the couple had separated in June, after Ben - the multi-millionaire son of the late financier Jimmy Goldsmith was taken away from the couple's Notting Hill home by police following an argument.
Then followed a war of words on Twitter, but the couple agreed to make peace.
In a joint statement released through a PR company at the time, they said: "It is a matter of regret to us that, at a time when our emotions and those of our friends have run high, things have been said in public which should have been kept private.
"We accept our full share of responsibility for this. There will be no further comment, either directly or indirectly, from us on any aspect of our family's private life."
The pair have a daughter, eight-year-old Iris Annabel, and two sons, seven-year-old Frank James Amschel and four-year-old Isaac Benjamin Victor.
Music producer Kate, the heiress of the Rothschild banking dynasty, isn't the only one to have moved on.
Ben, 31, has started dating 25-year-old film producer Jemima Jones.