Emily Arbuthnott, 37, struck James Lyall, 82, while he was walking across the road in front of her car at a zebra crossing on the A92 in Inverbervie in November 2014.
Mother-of-three Arbuthnott, of Laurencekirk, admitted the charge at Aberdeen Sheriff Court.
Sentence was deferred for background reports.
The charge stated the driver failed to slow down or stop when her vision was obscured by the sun and did not notice Mr Lyall as he crossed the road.
He was taken to hospital in an ambulance under police escort but later died.
Arbuthnott, the daughter-in-law of the 17th Viscount of Arbuthnott, was not injured in the crash which happened on 5 November 2014.
A motorist has admitted causing the death of a pensioner by driving carelessly on an Aberdeenshire road.