Kosinski began work in broadcast journalism in Rockford, Illinois for WIFR while earning her BA and MA from the Medill School of Journalism at Northwestern University. Leaving WIFR, she moved to Charlotte, North Carolina at WSOC-TV and founded the Piedmont Bureau. In 2001, she was voted the Best Reporter in Charlotte by readers of the city's arts and entertainment magazine. In Fall of 2001, she left WSOC-TV for WTVJ in Miami. She is a 2003 Suncoast Regional Emmy Awards Craft Winner for reporting on Haitian immigrants and was named Woman of the Year in 2005 by Women in Communications of South Florida.
Kosinski has covered such world events as the War in Afghanistan, terrorist plots in Europe, international court cases, and the 2010 Haiti earthquake.
In 2009, she won a national Emmy award for live reporting during NBC News' special coverage of the 2008 U.S. presidential election.
In 2010, Kosinski was named a foreign correspondent and moved from NBC's Southeast Bureau to London.
In 2014, she became a White House correspondent for CNN.
In 2005, Kosinski worked as a foreign correspondent for NBC News based in London. She covered numerous international events such as the 2010 Haiti earthquake, the aftermath of the Arab Spring, the assassination of Benazir Bhutto, terrorist plots and bombings in Europe, US-Russia relations, and Virginia tech shooting.
In August 2005, while covering the Natalee Holloway disappearance in Aruba, NBC arranged an interview with the director of Aruban prisons and a tour of suspect Joran van der Sloot's prison. During the tour, she ran into van der Sloot and conducted an off-camera interview. Prison director Fred Maduro appeared live on MSNBC and admitted that he offered her the tour. After Van der Sloot's attorneys called for a hearing on the matter, NBC declined to air the footage.
In 2009, Kosinski purchased a $5.6 million, 12,000-square-foot (1,100 m2) home in Coral Gables, Florida.
Kosinski attended St. Charles Borromeo School and graduated from Holy Cross High School. She earned bachelor's and master's degrees from the Medill School of Journalism at Northwestern University. Originally in the magazine writing program, she later switched to the broadcasting division.
Kosinski was born in Willingboro Township, New Jersey, and grew up in nearby Cinnaminson Township, New Jersey, a suburb of Philadelphia. She is one of four children of Jeanette, a chemist, and Robert, a retired biologist who worked for the New Jersey Department of Environmental Protection. While working as a television news photographer and reporter in Charlotte, North Carolina, she also performed with the award-winning Piedmont Players theater group, in roles such as Elvira in Blithe Spirit and Suzanne in Don't Dress for Dinner. As of 2011 she is living in London.
In August 2013, Kosinski announced her engagement to retired investment banker and philanthropist Kimbell Duncan. They married on August 9, 2014.
For several years, Kosinski has served on the executive host committee for Amigos For Kids, a charitable organization based in Miami. Kosinski has volunteered for and supported numerous community organizations. She has sought out and highlighted innovative charitable work around the United States, as a regular contributor to Nightly News' "Making a Difference" series, which has in turn positively impacted those efforts.