Finlay theorized that yellow fever was spread by mosquitoes. He completed his education in Havana and Paris before settling in Cuba, where he initiated medical practice.

Finlay was appointed to work with a North American commission to pursue the study of the causes of yellow fever in 1879. He was sent to the fifth International Sanitary Conference in Washington DC as a Cuban delegate. 

The Cuban scientist sought to persuade International Sanitary Conference of his mosquito-vector theory in 1900 when US Army's Yellow Fever Board arrived in Cuba. His hypothesis was confirmed by the board's head, Dr Walter Reed, of the US Army.

Finlay’s discovery largely helped in starting the process to help the cause of eradication of yellow fever and served as a life saver to many. He passed away in August 2015 from a stroke caused by severe brain seizures at his residence.