Charlton missed out on spot in the League One play-offs despite beating Hull 1-0 on Sunday.
The Addicks went into the final day outside of the top six and needing both Portsmouth and Oxford United to slip-up to have any hope of leapfrogging their rivals.
An own goal from Jacob Greaves gave Charlton victory against Hull at the Valley but they finished outside the play-offs on goal difference after Oxford beat Burton 4-0 to secure the final top-six place. Portsmouth lost 1-0 against Accrington.
It means a disappointing end to the season for Charlton, who now face another season of rebuilding in League One under Nigel Adkins.
Charlton started the day in eighth position, one point behind Oxford in seventh and two points off Portsmouth in sixth.
Oxford raced into a 2-0 lead in the first half against Burton and it was all over for Charlton when Oxford went four goals to the good in the second half.
Charlton did secure three points late on against Hull when a Liam Millar cross was diverted into his own net by Greaves but it was all in vain.