Poysden and Mellor, both 25, have committed to the Bears until at least the end of the 2018 season after five Championship games each this summer.
Warwickshire chief executive Neil Snowball said: "Josh and Alex have big futures here at Edgbaston.
"They played important roles in 2016 and proved that they can compete at the highest level of the domestic game."
Poysden, who was already tied to Edgbaston to the end of 2017, has worked with the England Lions at the National Cricket Centre in Loughborough this autumn and is currently with the Lions squad at their training camp in Dubai.
He has made 16 one-day and 19 T20 appearances for the Bears since initially joining on trial after leaving Cambridge University ahead of the 2014 season.
He also made a late-season breakthrough in the County Championship this summer, taking 15 wickets in his five appearances, including a career-best 5-53 against champions Middlesex at Edgbaston.
Former Staffordshire player Mellor, who first joined the Bears at the end of the 2015 season on a one-year deal, has now signed a new two-year contract.
Mellor, whose late-season Championship appearances came as cover for the injured Tim Ambrose, has also played three limited-overs games.
Warwickshire are still without a director of cricket following the departure of Dougie Brown in October.
They have been strongly linked with a return to Edgbaston for former Bears boss Ashley Giles, who is still contracted to his current county Lancashire for another year.
BBC Sport understands that negotiations remain ongoing with Lancashire over Giles, who still lives in the Midlands.
Chief executive Neil Snowball reiterated last week that they would like to make an appointment before Christmas.
Warwickshire leg-spinner Josh Poysden and wicketkeeper Alex Mellor have signed contract extensions.