In 2003, at age 15, Alonzo joined the TV drama Kay Tagal Kang Hinintay. She later appeared in soap operas like It Might Be You, Ikaw Ang Lahat Sa Akin, Maging Sino Ka Man and Maging Sino Ka Man: Ang Pagbabalik.
In June 2008, Alonzo released her debut album The Real Me under Star Records. It was also in mid-2008 when it was announced that she would play the leading character Betty Pengson in the TV sitcom Yo Soy Betty La Fea, which originated from Colombia. The Philippines' version was entitled I Love Betty La Fea. The show premiered on September 2008 at the primetime slot; it ran for six consecutive months.