The 28-year-old footballer was jailed in March after being convicted of engaging in sexual activity with an underage fan.
He had already admitted grooming and kissing her in a sexual manner.
His partner and mother of his child, Stacey Flounders, tweeted picture reading "happy birthday daddy" in multicoloured letters.
Johnson was sentenced to six years at Bradford Crown Court. Earlier this week we reported how he lost an appeal.
His sister Faye posted on social media: “Absolutely devastated that the single judge has rejected my brother’s appeal against sentence and conviction.
“But I can assure every supporter that it has made me stronger and I will never give up.”