And the member for Lancaster and Fleetwood member marked her departure by firing off parting shots over Jeremy Corbyn's continued exclusion from the Parliamentary Labour Party and what she sees as the opposition's failure to push for proportional representation at the ballot box.
Ms Smith insisted she had intended to leave her role as Shadow Secretary of State for Young People and Democracy at a scheduled meeting with Sir Keir on Wednesday and "just brought things forward 48 hours."
At the same time she announced she intends to remain in the Commons as a backbench MP - and still plans to stand at the next General Election.
The staunch supporter of Labour's previous leader told followers on social media: "I’m looking forward to spending even more time at home here in Lancashire and standing up for my constituents."
And she revealed she had turned down an offer to remain in her role as part of the new look front bench team.
In her letter of resignation she told Sir Keir: "It has been an honour to serve in the Shadow Cabinet since June 2016 and on the Labour front bench since September 2015.
"Although I am grateful for your offer to remain in my current brief, I have only been on the backbenches for four months in my six and a half years as an MP and I will instead be returning to the backbenches.
"I wish to focus more of my time in my Lancashire constituency in advance of the next election which I look forward to contesting.
"I have always considered myself a team player, focused solely on the intention of electing a Labour Government which my constituents, and people right across Britain, desperately need.
"You will be aware that we had a meeting scheduled for later this week, during which I wanted to raise the issue of, and my concerns about, Jeremy not being readmitted to the Parliamentary Labour Party after he was readmitted into our party membership following due process.
"This position is utterly unsustainable and it is important that you truly understand how much damage this is causing in Constituency Labour Parties and amongst ordinary members.
"You will also be aware that I am a longstanding advocate of proportional representation and I am disappointed that we, as a party, have not adopted a position which I believe to be fundamentally fairer and very much in keeping with the views of many of my constituents,
"In returning to the backbenches I wish your new Shadow Cabinet well. However I do hope that you will reflect on some of these concerns I have aimed to outline constructively and from the point of views of one of our few remaining northern 'red wall' Labour MPs.
"In the meantime I look forward to spending more time in Lancashire, campaigning for proportional representation and challenging the Government on their woeful inaction on tackling the climate emergency."
Ms Smith, 36, became an MP in May 2015, beating Eric Ollerenshaw for the Lancaster and Fleetwood seat. She had been previously unsuccessful for Labour in Wyre and North Preston in the 2010 election.
Labour's deputy leader Angela Rayner said: "I want to thank Cat Smith MP for all of her hard work in my team, especially in exposing and standing up against the Conservatives' anti-democratic power-grab Election Bill.
"Cat is a brilliant local MP and I know she will continue to stand up for her constituents from the backbenches."