Hey, good guess!
Mormons believe that the individual known as Jesus Christ in the New Testament is the same individual as the individual known as Jehovah in the Old Testament. In other words, in Exodus 3:14, which reads, "And God said unto Moses, I AM THAT I AM: and he said, Thus shalt thou say unto the children of Israel, I AM hath sent me unto you," Jehovah was speaking. And in John 8:58, when we read, "Jesus said unto them, Verily,verily, I say unto you, Before Abraham was, I am," the speaker is clearly Jesus Christ. So we believe that the pre-mortal Jesus was known as Jehovah to the people of the Old Testament, was born to Mary and became known as Jesus.
Don't misunderstand me, though. We do not believe that Jesus is the same individual as God the Father, and we don't believe that God the Father was the individual spoken of in the Old Testament as Jehovah. When we call Him anything other than "God" or "our Father in Heaven," we would use the term "Elohim."
There are actually numerous examples I could give of there Jehovah and Jesus are described in exactly the same way, but because of all the verses implying that they are two physically distinct individuals, we see these many similarities between Jehovah and Jesus as evidence that they were the same person, rather than as evidence that the Father and the Son are somehow two different manifestations of the same substance.