Monicka

After portraying a supporting role as the protagonist's sister in the Hindi film Kaalia (1997), Castelino made her debut in a lead role through K. S. Ravikumar's 1999 Tamil romantic thriller film Minsara Kanna, appearing opposite Vijay in a leading role. Simran was the first choice to play as lady lead, but due to her other commitments she opted out and Monica was subsequently signed on. She shot with the team in Chennai, Switzerland and Germany, notably filming the "Un Per Solla" song in the Alps. The film opened to average reviews and collections at the box office, with the actress gaining mixed reviews for her work. She briefly changed her stage name to Anjani/Madhuri for numerological reasons, before reverting to Monica.

Her last successful movie, "Mo mana khali tori paen", was opposite Sidhant Mohapatra. Failure to get further opportunities saw Monica play a glamorous lead role in a low budget Hindi adult film Kama Sundari, before playing a similar supporting role in Meri Partigya, which had Mithun Chakraborty in the lead role. She was later seen in an item number in Shikaar (2004), performing a club dance song in the climax of the film. In 2004, she signed on to work on Men Not Allowed where she portrayed a lesbian woman in love with a woman portrayed by Payal Rohatgi. She then continued to play glamorous roles in Hindi films in the late 2000s.

In the 2010s, Castelino moved on to portray roles in television serials and notably featured in Tu Mere Agal Bagal Hai (2014), Dr. Madhumati On Duty (2016) and Gupp Chupp (2016) in lead roles. She regularly also worked on shows produced by Optimystix Entertainment. In 2017, she joined the Hindi comedy programme, The Kapil Sharma Show, as a regular cast member.