Garcia shook off two early bogeys with two birdies on a flawless back nine to card an even 71 and finish with a 6-under total of 278 at the Valderrama course.
"It's very, very special. Valderrama ... I have so much history here and unfortunately it wasn't as good as this until now," said Garcia, who had previously finished runner-up three times on the course.
Jimenez seized the lead early when Garcia three-putted the sixth hole and bogeyed the next. The 47-year-old Jimenez dropped shots on the 13th and 15th before making a pair of birdies to press Garcia until the last hole.
Jimenez had a chance at pulling even with a birdie on the 18th, but his 15-foot putt fell short and Garcia calmly made par.
"On 18, we decided to play a little more conservatively and make sure we hit the fairway," Garcia told the European Tour website. "I've been in that situation three or four times on this course. It wasn't easy, but I believed in my ability. To be able to hit the chip I hit there and roll the putt in with the pressure, it was nice."
The 31-year old Garcia won the Castello Masters by 11 strokes last weekend to end a nearly three-year title drought.
"It's been two amazing weeks," said Garcia, who became the first Spaniard to win a stroke play event at the cork tree-lined Valderrama course. "I wasn't as good as probably the last 13 days, but we hung on and managed to pull through."
Once ranked No. 2, Garcia had dropped outside the top 75 after near-misses at the 2007 British Open at Carnoustie and 2008 PGA Championship at Oakland Hills.
His fifth European Tour win on his home soil will lift him back into the top 20. It also qualified him for next week's WGC-HSBC Champions in Shanghai and moved him to the top of the European Ryder Cup points race.
"Ryder Cup years are always special for me," he said. "So hopefully we can make that team."