Andrelle Leedham faces a lengthy sentence after masterminding the plot.
Lovestruck warder Hannah Stewart, 27, fell for the 26-year-old father-of-one while he was locked up for blackmail, Stafford Crown Court heard.
The pair met in the education department where Stewart was an operational support officer.
Prosecutor Mr Gerard Quirke said: "He showed her a lot of attention, using the charm he undoubtedly has."
Recorder Peter Cooke said he believed the lovers had sexual relations in the prison, which sits alongside the M54 and serves Shropshire.
Stewart had maintained they were not intimate until after Leedham was released in August last year. The sexual relationship fizzled out within a couple of months but they remained in contact and the defendant hatched the drug plot by text message.
He arranged for her to take a sports bag containing cannabis, 700 anabolic steroid tablets, eight bottles of spirits and seven mobile phones into the jail. The plot was foiled when an inmate was caught with the bag just after the handover.
Stewart, from Oaks Drive, Featherstone, whose father and brother have both been prison officers at the jail, has admitted her involvement in the drug plot and misconduct in a public office. She is awaiting sentence.
A jury unanimously found Leedham, who was living in a bedsit in Telford after his release, guilty of conspiring to supply cannabis and anabolic steroids and conspiracy to convey prohibited articles into HMP Featherstone.
Recorder Cooke said: "He inveigled his way into her affections and was manipulative. He was threatening to turn up on the doorstep of her family and announce what had been going on, which would not have gone down well with her father and brother."
Leedham was remanded in custody and will be sentenced with the two inmates also involved in the plot at a later date.