Cullen, a former player for the national hockey team in her native New Zealand, had been Hamilton's dedicated performance coach since 2016 as part of her role at the Hintsa athlete coaching company that has long provided coaches for many F1 drivers.
As a duo, Hamilton and Cullen became a regular sight in the F1 paddock, where the latter was a popular figure.
Cullen was typically one of the last people to interact with the seven-time world champion before he climbed aboard his various Mercedes cars in recent years.
Writing on his Instagram account where he announced the news he and Cullen will no longer be working together, Hamilton stated: "For the last seven years, [Cullen] has been by my side, pushing me to be the best version of myself.
"I am a stronger athlete and a better person because of her. So, today I hope you'll join me in wishing her the very best as she takes her next steps to pursue her dreams.
"Thank you for everything Ang, I can't wait to see what the future has in store for you."
Cullen also paid tribute to her work with Hamilton on her Instagram account.
"Exactly seven [years] ago on this day I was standing in the F1 paddock for the first time at the Australian GP," she posted. "Today I'm excited to share I'm off on my next adventure.
"I am so grateful and blessed to have had this incredible journey in F1 and I know my story will continue.
"Thanks to the [Mercedes] team, who have been my family for the past seven years.
"And Lewis Hamilton you GOAT [Greatest of All Time]!! It's been such an honour and pleasure to stand alongside you I'm so proud of you and everything you have achieved.
"Thank you for supporting me, believing in me and showing me the limitless potential we all have within us.
"I am so excited to watch the next chapter for you. There's nothing you can't do."
Cullen, who accompanied Hamilton to the Bahrain test and race events that opened the 2023 season, is yet to reveal what her next project will be.