Torrential rains began in the afternoon, and emergency officials said they evacuated more than 4,250 people in coastal towns and set up 58 shelters ahead of the unsafe storm.
Willa was centered about 75 miles (120 kilometers) east-northeast of Durango, Mexico, and heading to the northeast at 25 mph (41 kph).
Forecasters said Category 3 Hurricane Willa made landfall on Mexico's coast near the town of Isla del Bosque in the state of Sinaloa.
Two young men smoke and play their guitars as they wait for the landfall of Hurricane Willa in Mazatlan, Mexico, Tuesday, Oct. 23, 2018.
Willa was expected to bring life-threatening storm surges, rain and wind to residents on Mexico's Pacific shore, according to the US National Hurricane Center.
Farther south, Mexican officials reported late Monday that there have been 12 deaths related to heavy rains from Tropical Storm Vicente.
There were no immediate reports of deaths, but the storm's 120 miles per hour (195 kph) winds knocked out power and damaged buildings in the Sinaloa state municipality of Escuinapa, and local news media published images of a damaged hospital.
It is aiming at a low-lying region where towns sit amid farmland tucked between the sea and lagoons. A decree of "extraordinary emergency" has been set for 19 municipalities in Nayarit and Sinaloa states.
The storm also battered the Islas Marias, a group of Mexican islands about 100km off the mainland that include a nature preserve and a federal prison.
Willa came ashore about 50 miles (80 kilometres) southeast of Mazatlan, a resort city that is home to high-rise hotels and about 500,000 people, including many US and Canadian expatriates.
The western states of Sinaloa, Jalisco, Colima and Nayarit are expected to be most affected by Willa, which officials said would likely hit in the evening.
"It was really strong", said Cecilia Crespo, a police spokeswoman in Escuinapa, a seaside town near where the storm plowed inland.
"The people don't want to evacuate, but it's for their security", he said.
Alberto Hernandez, a hotel worker in the town of Teacapan close to where the storm made landfall, said he and his son, who also works at the hotel, were staying on the job, though the rest of his family had left the area.
It is now moving north-eastwards across Mexico where it is expected to lose strength rapidly, and is likely to become a tropical storm or depression on Wednesday. American Airlines canceled all flights from Mazatlan.
Downpours in Mexico prior to Willa's arrival have heightened the risk of flooding, and the NHC said the storm could drench some areas in as much as 18 inches (45 cm) of rain.
Officials in Oaxaca state said seven adults and five children had lost their lives in drownings or mudslides.