Yes, because non-Catholics are not bound by the laws of the Church regarding marriage. The Church recognizes marriages between non-Catholics. As long as there were no impediments (prior marriage, for example) a marriage between two non-Catholics is valid no matter where it occurred. So two atheists who marry at city hall contract a valid marriage in the eyes of the Church. A baptized Protestant who marries an unbaptized atheist at a Protestant church contracts a valid marriage in the eyes of the Church.
Two validly baptized Christians (of any denomination) contract a valid, Sacramental marriage. Because the Church recognizes Protestant baptisms as valid (providing they used the correct form, used water, had proper intent), the Church recognizes two baptized non-Catholics to be Sacramentally married. So two baptized Lutherans who marry in a Lutheran Church are sacramentally married. Two baptized Baptists who marry at city hall are sacramentally married.
If any of the hypothetical couples above were to convert to Catholicism, nothing would need to be done concerning their marriages, because the Church already recognizes them. As I said before, this is assuming there were no prior marriages/divorces, or impediments.
To share my own experience, I was married to my husband, who was baptized in the Baptist church, at our non-denominational evangelical church. At the time of our marriage, I was not baptized. We contracted a valid marriage in the eyes of the Catholic Church.
A year later, I was baptized in the same church. At the moment of my baptism, our marriage became sacramental.
Two years later, I converted to the Catholic Church. No further steps had to be taken regarding my marriage, because it was already sacramental and valid in the eyes of the Catholic Church.