Born in San Juan, Puerto Rico. A graduate of the University of Puerto Rico, she studied with Rosa Luisa Márquez, Dean Zayas, Victoria Espinoza, Petra Bravo, and Viveca Vázquez, among others. As part of the UPR Traveling Theater, Casiano participated in the innovative staging of plays like Octopuses Garden, The House of Bernarda Alba, Absurdities in Solitude, and Procession. She also appeared in Marquez's acclaimed version of Waiting for Godot, featured in the Ayacucho '98 Theater Encounter and the International Theater Festival in Lima, Perú. 

In 1996, Casiano co-founded Colectivo C-4, one of Puerto Rico's most important experimental theater groups. In 2001, C-4 was part of the 10th Havana International Theater Festival. In 1998, Casiano moved to Ecuador to study with the renowned group Malayerba. She returned to Puerto Rico and wrote, directed and performed her first one-woman-show,¿Qué me trajiste?: Cabaret Boricua, which sold out in New York in 2003. At the end of 2000, the National Endowment for the Arts and the Institute of Puerto Rican Culture granted her funding to produce her second one-woman-show,Colonia 2007 o el cabaret de los días terribles, which accompanied ¿Qué me trajiste? on tour through Ecuador, Perú, Chile and Argentina in 2001, and was also a success at HERE Arts Center's The American Living Room 2004 festival in New York. 

Casiano has worked off-Broadway with companies such as Repertorio Español starring in Blood Wedding, The House of Bernarda Alba, Bad Blood, and The Dog in the Manger (ACE 2005 Best Comedy Actress nominee) among others, and the Puerto Rican Traveling Theater in La Lupe: My Life, My Destiny as La Lupe’s understudy. In 2003, she starred in the film The Love Bite, which won Best Irish Short at the Galway Film Fleadth 2004 in Ireland. Casiano worked with The Flying Machine at the Public Theater, and trained with the Atlantic Theater  Company, the Michael Chekhov Acting Studio and The Second City in New York. In the summer of 2005, she starred as Medea in a site-specific production of the Greek tragedy at El Morro Castle in San Juan to great reviews. In May 2005, she co-founded La Criatura theater company.Their first project, the site-specific play Silence Is Health/Silencio es Salud was written, directed and starred by Casiano and premiere in February 2006 with a reprise by popular demand that summer. 

Karina has trained with Penny Templeton, Hank Schob, Jack Poggi, Karen Fuhrman, Kevin Scott, Michael Rock and B. Lynne Jebbens, among others. In late 2006 she played lead roles in the short films Trigger Effect by Crucible Films shot entirely in Washington D.C,  That's Good, That's Enough by FemmeMaker Productions and Mojado by CT Pictures (featured in the Santa Barbara International Film Festival 2008.) In January 2007, Casiano joined Aya Ogawa's Knife Inc. theatre company in their production of ophelia3 at HERE Arts Center. And in April 2008 she appeared in the premiere of Charles Mee's Fire Island with 3-Legged Dog. That same month, Casiano premiered (on 42nd Street in Manhattan) her third one-woman show entitled Rootless: La No-Nostalgia, a theatrical multimedia concert about the effects of migration on individuals. Rootless will begin touring immediately in the U.S. and other countries. Her collaborators for this show include singer Gwen Conley and musician Dean Bohana and jazz pianist Harry Miller.