McGee, 32, was in the starting team for Saturday's All-Ireland quarter-final defeat by Dublin while forward McFadden, 33, was a late substitute.
McFadden played a record 170 games in a Donegal career which reaped an All-Ireland win and All Star award in 2012.
McGee was also in that All-Ireland winning side and made 154 appearances for the Ulster county.
St Michael's club player McAadden told the Irish Times: "I don't know was it 14 or 15 years? The game is changing all the time, and there is plenty of fine talent coming through, so it is time for them to take up the baton.
"The future is bright for Donegal football."
There are bound to be questions about the inter-county futures of other experienced Donegal stars.
Midfielder Rory Kavanagh, who retired at the end of the 2014 campaign and is 34 next week, came back this year but failed to produce his best form.
Neil Gallagher, another midfielder, missed this year because of injury while half-forward Christy Toye, a sub against Dublin on Saturday, could have played his last match.
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Defender Eamon McGee has joined Donegal team-mate Colm McFadden in announcing his inter-county retirement.