Ultimo revealed Ms Mone left its board and the board of parent group MJM International earlier this month.
Recently, she was appointed by the government to carry out a review into how best to encourage start-ups in areas of high unemployment.
Ultimo said new projects and commitments had left limited time for her engagements with the firm.
Glasgow-born Ms Mone, 43, co-founded the original Ultimo parent company MJM International with her ex-husband Michael in 1996, and created brands including Ultimo Miracle Shapewear and Miss Ultimo.
She sold an 80% stake in the firm last year to Sri Lanka-based lingerie group MAS Holdings in a multi-million pound deal.
In a statement, Ultimo said: "Ultimo wishes Michelle every success in her new personal projects and public engagements, and thanks her for contributions in the past and for her future support of Ultimo's growth."
Ms Mone has a high media profile and has appeared on TV shows as diverse as The Apprentice, Celebrity Masterchef and Surprise Surprise.
She also owns stakes in a tanning firm, a slimming business and works as a motivational speaker.
Entrepreneur Michelle Mone has resigned from the board of Ultimo, the lingerie and swimwear firm she founded in 1996.