[Relief depicting Cleopatra I at El Kab, Egypt, photographed 2010, source: Wikimedia Commons]
Cleopatra was the daughter of Antiochos III and (most likely) Laodice III. It’s likely that she was their third or fourth child and probably second daughter. Her father announced her betrothal to Ptolemy V in 196 BCE. The announcement itself was undoubtedly a political move. He was hosting Roman envoys at the time and Rome supported the Ptolemies over the Seleukids.
The actual marriage took place in 194 or 193 BCE, two or three years later, and Cleopatra joined her husband in Egypt. Here she was named Syra as a reference to where she had come from. Almost immediately, Ptolemy had her incorporated into the dynastic cult, granting her the same religious honors he held. They had at least three children together: Ptolemy VI, Cleopatra II,* and Ptolemy VIII.
Ptolemy V died in 180 BCE, leaving Cleopatra I and Ptolemy VI as co-rulers. In practice this meant that Cleopatra ruled alone as regent, as her son was only six at the time, and as such was named before him in all official documents. She minted coins with her own image and dealt with the precarious diplomatic situation between Egypt and the Seleukid Empire. In the last years of his reign, Ptolemy V had been planning war against is brother-in-law. On her ascension as regent, Cleopatra put an end to those plans, preferring the diplomatic route. She died in 178, the first Ptolemaic queen to rule on her own.
*Some scholars are reluctant to definitively consider Cleopatra II as Cleopatra I’s daughter because she is never named as such, unlike her brothers. Considering, though, that Cleopatra II is named as Ptolemy V’s daughter, that she had to have been legitimate, and that Ptolemy V never had any wife but Cleopatra I, there really is no other option.