He was first elevated to Cardinal-Deacon in December 1198 by his cousin, Pope Innocent III. In 1206 he was made Cardinal Bishop, and named Dean of the College of Cardinals in 1218. At the request of Francis of Assisi, Pope Honorius III made him Cardinal Protector of the Franciscans. (He had been a friend of Francis already, as well as of Clare of Assisi and St. Dominic.)
Honorius died on 18 March 1227 after trying to establish the Fifth Crusade that had been called by Innocent III. Innocent intended for this one to be led by the papacy, in order to avoid the disaster of the Fourth Crusade. Cardinal Ugolino was elected pope one day after the death of Honorius, taking the name "Gregory" because he was at the monastery of St. Gregory when he accepted the position.
One of his first acts was to expand the powers of an inquisition taking place in Germany. He also established a Papal Inquisition (mostly managed by Dominicans and Franciscans) to formalize what had already been begun and was being handled differently across Christendom. His aim was to introduce due process and objectivity, because too often executions were done by unruly mobs on the innocent in the name of defeating heresy.
He also called for Crusades in places other than the Holy Land, to bring Eastern Europe into alignment with the papacy. These Northern or Baltic Crusades were against the pagan Baltic, Finnic, and West Slavic peoples.
Some modern writers blame Gregory for the Black Death because of a bull he wrote that demonized cats. The widespread killing of cats (the thinking goes) removed a deterrent to the rats that spread the plague. What the proponents of this theory leave out, however, is that papal decree does not run to India and China where the plague was just as widespread as in Europe. Also, one would have to assume the killing of cats was consistent for over a century, since the plague arrived in Europe about 120 years after Gregory's bull.
We will not condemn every act of Gregory: In 1229, when the University of Paris had a strike, he wrote a bull that helped resolve the differences between Town and Gown.
One of his lasting achievements, however, was to institutionalize anti-semitism in the Church. For that, we will wait another day. See you tomorrow.