She was the, or a, power behind the throne during the reign of François II, who was technically of age when he succeeded, then formally Regent during the first three years of Charles IX. When he was declared of age, still just thirteen years old, she carried on running things, but was back to pulling the strings unofficially rather than being official ruler. This continued after the accession of Henri III, even though unlike his brothers he was fully adult when he succeeded.
So much for Catherine, for whom, like Henri IV, I have some sympathy despite all her misdeeds; she was a devoted mother at least, and did all she did trying to do her best for her sons and France. It was a very tough row she had to hoe. She is also credited with being the founding mother of French cuisine, the culinary practices she imported from Italy transforming and setting it on the path to renown. She was a considerable patron of other arts too, and the beginnings of ballet can be traced to her endowments and encouragement.
Back to Henri II and living longer. He was a vicious persecutor of Protestants, and Catherine as a practical matter was inclined to be more tolerant, though all efforts at accommodation failing due to the intransigence of Catholic and Protestant parties alike her long period of effective rule was stained by atrocities, the St Bartholomew's Day massacre being the most infamous of course. Ironically, it is possible to see a prolonging of Henri II's reign as averting the French Wars of Religion, or at least reducing their scale and impact. The Guises would have stayed on the sidelines and there would never have been the need to form a Catholic party as such, since the King was already doing all that they might.
And without the hope offered by Catherine's initial willingness to be reasonable and bargain, the Huguenot leaders might never have gotten into a position of enough strength for civil war to ignite, also as they would be facing an undivided establishment behind the King, rather than one fragmented between the Guises and the Queen Mother. That's one way things might have gone, anyway.