Baroness Thatcher marked the occasion by having lunch at Green's Restaurant and Oyster Bar in London's exclusive St James's district with her son Mark and his wife.
Writing on Twitter, Mr Romney - the Republican challenger engaged in a close fight with Democrat Barack Obama for the White House in next month's election - said: "The US is blessed with courageous friends.
"Happy Birthday to the Iron Lady, Margaret Thatcher - a tower of strength in the cause of liberty."
It drew criticism from Prime Minister David Cameron and others for concentrating on the peer's failing health - a series of small strokes since leaving office have left her frail and with dementia.
She was not sufficiently well to join the Queen for a lunch with former and serving prime ministers as part of the Diamond Jubilee this summer and previously missed her own 85th birthday party thrown by Mr Cameron at 10 Downing Street.
Her last public appearance was at another birthday party, in September last year, when she helped then defence secretary Liam Fox celebrate his 50th.
A spokesman said she was "doing all right".