Kate Tsui was born and raised in Hong Kong. Her father works in logistics, and while her mother is now a homemaker, she was a dancer when she was younger. Tsui also has an older sister, who is married with two kids. As of 2014, Tsui has expressed that with the exception of herself, her entire immediate family now resides in Taiwan.
Prior to winning her beauty queen title, Tsui had originally aspired to become a professional dancer. She had begun practicing ballet since age 4, but was forced to quit when she was 11, due to an injury on her right ankle. At age 14, she had begun to practice Hip Hop and Jazz dance. She is skilled in different interests such as acting, dancing, learning languages, and snowboarding.
In 2004, she entered into the Miss Hong Kong beauty pageant and was crowned as the winner; in the pageant, she had also received Miss Photogenic, Miss International Goodwill, and the Slimming Beauty Award. After the pageant, she had signed with TVB and began her career as an actress.
While Tsui's TV career quieted down during 2009 to 2010, due to her focus on her film career, 2011 proved to be fruitful for Tsui, with six of her TV dramas airing on TVB. Of the six roles, Paris Yiu from Lives of Omission had garnered the most attention. For the role, Tsui was awarded a My Favourite TVB Female TV Character award from the StarHub TVB Awards, nominations in the Best Actress and Favourite Female Character categories for the TVB Anniversary Awards 2011, as well as a nomination for Outstanding Actress in Television by the Ming Pao Anniversary Award.
2012 is arguably Tsui's peak year in her TV career. In Highs and Lows, Tsui portrays an extremely tragic character that struggled with substance abuse, gang rape, and eventually, drug dealing. Tsui described her experience of filming this drama as "being on an emotional roller coaster", to the extent that she did not wish to speak to anyone or pick up the phone after work. The producer of the drama, Lam Chi Wah, referred to Tsui's role in Highs and Lows as the greatest breakthrough in her career and applauded her for setting down her image as a beauty pageant winner to dive into such a complex character in the drama.  Film director, Patrick Kong, also noted in his column that Tsui's acting has greatly matured in Highs and Lows, particularly in her portrayal of emotional scenes. 
In 2014, Tsui took part in a TVB-produced micro film, A Time of Love, which consists of four individual stories, each with a separate theme. Tsui starred opposite Taiwanese actors, James Wen and Chris Wang, in the "sorrow" themed story. After the airing of the micro film, TVB received 39 individual viewer statements that commended Tsui of her outstanding performance in the micro film. Column writer, Ko Leung, of Macao Daily, also stated Tsui portrayed her character in the micro film perfectly and that Tsui's acting is comparable to that of Bai Baihe in The Stolen Years.
In 2015, Tsui's management contract with TVB ended. However, Tsui said that she is still on very good terms with TVB, and TVB has settled job arrangements for her up until February 2016. Despite having received several offers, Tsui stated that she will not finalize any management contract agreements until she has completed her jewelry design courses.
Aside from her development in television acting, Tsui also experienced success in developing her career in the film industry. In 2007, through a series of auditions, Tsui was cast as the female lead, alongside A-list actors Tony Leung Ka-fai and Simon Yam, in Eye in the Sky, which is a film produced by Johnnie To and directed by Yau Nai-hoi. With her performance in the film,
In 2011, Tsui reprised her role as Paris Yiu Ho Ho from the TVB series Lives of Omission in its film sequel, Turning Point 2, in which she starred opposite Francis Ng. Playing a character with a mental disorder in the film, Tsui said that collaborating with Ng was a valuable experience because Ng was extremely willing to teach and give her suggestions about her acting and performances. From working with Tsui, Ng had openly praised her for her acting potential.
In 2013, Tsui took part in a comedy film, I Love Hong Kong 2013. It is also the first film that veteran actress, Veronica Yip, has taken part in since her retirement in 1996. In the film, Tsui plays the younger version of Yip's character. The producer of the film, Eric Tsang, specifically praised Tsui for providing the best performance out of the entire cast, referring to it as a "Best Actress performance".
In 2015, Tsui was cast in thriller film, Knock Knock, Who's There?. The film is the first directorial effort by veteran actress, Carrie Ng. Tsui said that upon receiving the script, she had intended to reject the role because of the frightening content. However, Tsui eventually agreed to the role due to Ng's persistence and persuasion. With reference to collaborating with Tsui, Ng said that Tsui is a very professional and admirable actress.
In December 2015, Tsui launched a jewelry line with K.S. Sze & Sons Ltd.. The collection is called "Rabbit-Duck Illusion". She indicated that her designs are inspired by Ludwig Wittgenstein's concept on the two different ways of seeing, using the ambiguous image of a "duckrabbit". She spoke of the concept behind her jewelry designs, "It's something that I like to remind myself of, that there are many situations in life that aren't worth splitting hairs over, and if I simply adjust the angle that I'm viewing things from, I can easily see a different perspective."