Ms Formby, who was appointed to the role as the party's general secretary a year ago, is set to start treatment this week.
In a statement released on Monday, Ms Formby, who lives in Southampton, said she would continue to work while she is being treated.
She said: “I am writing to you with some personal news. I have been diagnosed with breast cancer and will be starting my treatment this week. Although this news has come as a shock to me and my family, I feel so grateful that I have access to and support from our wonderful NHS.
My message to staff, NEC and PLP this morning. I’m feeling very positive thanks to our brilliant NHS.
Only Labour will end privatisation and outsourcing and improve our health service for everyone.#VoteLabour pic.twitter.com/D3uYYtHGAZ— Jennie Formby (@JennieGenSec)March 25, 2019
“I’m incredibly lucky that we have amazing teaching, research and women’s hospitals in Southampton. Breast cancer has had so much investment that outcomes are really positive, but I know that other cancers have less funding for research and treatment and that access to such good cancer care isn’t the same all across the country. I feel pride we are all working together for a Labour government that will change this.
“I will continue to work as much as I can. For those days when I can’t, the executive directors and my direct team will make sure everything carries on as usual.”
Shadow education secretary Angela Rayner was among Ms Formby’s first well-wishers following the announcement, writing on Twitter: “So sorry to hear this news, sending all my love and best wishes xx.”
Wes Streeting, the Labour MP for Ilford North, said: “I don’t fancy cancer’s chances against you! Wishing you the very best. We’re with you 100%.”
A Labour Party spokesman said: "Jennie Formby has told colleagues today that she has been diagnosed with breast cancer and will be starting treatment this week.
"Jennie will continue to work as much as she can, but is taking a realistic approach towards her treatment.
"She has a strong management team and on days that she is unable to work, Labour Party executive directors and the general secretary's direct team will ensure business as usual.
"The party is fully supporting Jennie through this period."