Holroyd, 30, arrives on a one-year deal from Macclesfield Town, having scored 13 goals last season - his best return in four years with the Silkmen.
Left-back Jennings, 29, played the second half of last season on loan at Wrexham from Cheltenham Town.
His performances persuaded boss Dean Keates to make the move permanent and Jennings joins on a two-year contract.
Wrexham have strengthened for next season by signing forward Chris Holroyd and defender James Jennings.