He says while he's determined to overcome the serious medical condition, he wants to be honest with everyone and he is just not physically able to continue, adding he's confident and optimistic about the future having found a way forward when faced with more hard times.
"But I want to be open and honest with my friends and colleagues in Sinn Féin, with the electorate of Foyle and with the wider community beyond my own constituency".
"Unfortunately, my health and the current crisis have overtaken this timeframe and I am stepping down from my role to make way for a new leader of Sinn Féin in the North", he said.
"This illness has taken its toll on me", he said, "but I am very determined to overcome it".
He said he could not pretend to agree with numerous positions Mr McGuinness has taken over the last 40 years, but "I believe he has sought to be a constructive force in trying to make the post-Good Friday Agreement institutions work".
The fallout between his party and Democratic Unionist Party was over the Renewable Heat Incentive (RHI), an initiative created to encourage businesses to use green energy.
Mr McGuinness' role bringing Sinn Fein grassroots along with him has been crucial in getting power-sharing up and running (or plodding, if you prefer).
Martin McGuinness, the former IRA commander who served for near a decade as deputy first minister in Northern Ireland, said Thursday he was quitting frontline politics to focus on recovering from a serious illness.
"I hope that Martin's political legacy - of a resilient and generous commitment to the interlocking institutions of the Good Friday Agreement - will encourage all of Northern Ireland's political parties to emulate his efforts to consolidate partnership government".
"As someone who has worked night and day over the course of 10 years to keep the institutions intact, and of course many conversations with both the British and Irish government about the DUP, it was particularly disappointing", McGuinness said. Uncertainty over the status of Northern Ireland's border with the Republic of Ireland after Brexit added to the political instability.
"As the Assembly election campaign gets underway, I continue to urge all parties to act responsibly in word and deed". "For my part, it was my intention to step aside in May this year", McGuinness, 66, said in a statement on Thursday.