According to About.com, a person can live with just one lung after a pneumonectomy. People who have a pneumonectomy must have adequate lung function in the remaining lung to support the person's breathing.
According to Live Science, if a person has a single lung, it inflates to fill up additional space in the chest cavity. Living with one lung can reduce exercise capacity but typically doesn't reduce life expectancy or the ability to perform everyday activities. One well-known person with one lung is Pope Francis I, who had a portion of his lung removed to treat an infection when he was 36.