LONDON — With smiles and backward glances, Prince Harry and his wife, Meghan, took part in their final engagement on Monday, honoring the British Commonwealth at Westminster Abbey, before they decamp for one of its frontier outposts, western Canada, and an uncertain new life as semi-royals.

As bagpipes played and cameras flashed, the couple walked in just before Prince William and his wife, Kate, and took their seats a row behind them. Meghan greeted her sister-in-law but the brothers scarcely spoke — their icy formality a metaphor for a once-close relationship that has all but unraveled.

When Queen Elizabeth and Prince Charles entered under the abbey’s vaulted ceiling to “God Save the Queen,” the familiar pomp was tinged with sadness. They, too, said little to Harry and Meghan, who once symbolized the future of the royal family and whose decision to leave almost ruptured it.

Harry has busied himself with a last series of appearances in recent days, including an Abbey Road recording session with the rock star Jon Bon Jovi and a travel conference in Edinburgh at which he told participants, “Just call me Harry.” Meghan paid a surprise visit to a classroom in London on Friday to mark International Women’s Day.